Bonding Character of Amino and Cyclopropyl Substituted Six‐Membered Ring Carbenes: DFT/NMR and AIM A

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Physical Chemistry Communications, Volume 3 Issue 2, October 2016 www.bacpl.org/j/pcc

Bonding Character of Amino and Cyclopropyl Substituted Six‐Membered Ring Carbenes: DFT/NMR and AIM Analysis Seyed Majid Musavi*, Javad Amani Department of Chemistry, Shahr‐e‐Qods Branch, Islamic Azad University, Shahr‐e‐Qods, 37515‐374, Tehran, Iran Corresponding author, Tel./fax: 98 21 46896519; E‐mail address: moosavi_majid@yahoo.com Abstract A systematic B3LYP/6‐311++G**//B3LYP/6‐31+G* calculations of GIAO/CSGT‐NMR (σd, σp, σii and ∆χanis) and AIM (

,

  and ε) are performed on some X‐substituted six‐membered ring carbenes (3a/3b, 4a/4b/4c, and 5a/5b; X=amino and 2

cyclopropyl). The findings confirm the interaction of divalent centers with neighboring attached groups, either amino or cyclopropyl, but have a much more pronounced impact for the former. The N–Ccarbene bonds of all singlet carbenes are stronger than their corresponding triplets and the π‐components also bring about higher charge transfer contributions as compared with the corresponding triplets. The large chemical shielding anisotropy at the Ccarbene could mainly be related to a low‐lying n→π* transition, σxx,p. The extent of paramagnetic contribution is generally proportional to the inverse of singlet‐triplet gaps ΔEs‐t. Calculated 13Ccarbene chemical shifts for singlet states are significantly more downfield as compared with their corresponding triplets. Calculated σiis at the Ccarbene show that the amino and cyclopropyl groups have the same behavior in the polarization of the X–Ccarbene bond toward X, but the amino group has a more π back‐donation ability. In the singlet carbenes, the absolute values of paramagnetic components decrease in the order of σxx,p > σyy,p > σzz,p, but in the triplet carbenes the order is almost reverse. The calculated ∆χanis values show that the electron delocalization to carbene center of all triplet carbenes is less than the corresponding singlet states. Keywords Six‐membered ring NHCs; 13C NMR; AIM; DFT‐B3LYP

Introduction Divalent carbon species, named carbenes, are important intermediates in organic chemistry [1‐4]. For a long time, this was a popular belief that carbenes are elusive species in the sense that they could only be directly observed by spectroscopic techniques, either in the gas‐phase [5‐9] or in low‐temperature matrices [10‐13], but they could not be isolated in macroscopic amounts at room temperature. Nevertheless, this idea was changed by the syntheses of stable crystalline singlet carbenes (imidazol‐2‐ylidenes 1 with R= adamantyl, aryl or tert‐butyl [14‐16], and imidazolin‐2‐ylidenes 2 with R= aryl [17]), all belong to the N‐heterocyclic carbenes (NHCs) family (Scheme 1).

R

R

N

N

N

N

R 1

R 2

SCHEME 1. TWO STABLE CRYSTALLINE SINGLET NHCS.

The discovery of transition metal complexes of NHCs has paved the way for NHCs to become universal ligands in organometallic and inorganic coordination chemistry. The stability of free NHCs has received much attention [18]. Besides, the experimental researches, the progress of computational chemistry in the last two decades has facilitated the study of such reactive species and has opened new insight into the characterization of different carbene species [19].

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